NO337017B1 - System and method for adjusting the gain of a gamma ray detector by gamma ray logging-under-drilling - Google Patents

Description

This invention is directed to the measurement of gamma radiation from an earth formation penetrated by a well borehole. More particularly, the invention is directed to the determination of concentrations of naturally occurring radioactive elements in soil formation by analysis of energy spectra measured by at least one gamma ray detector while the borehole is being drilled.

The measurement of naturally occurring gamma radiation as a function of depth inside a wellbore is the basis for one of the earliest geophysical borehole survey systems. This type of system, commonly referred to as a natural gamma ray logging system, typically includes at least one gamma ray detector housed in a downhole tool that is guided along the borehole.

One type of natural gamma ray logging system includes a logging tool that responds to total gamma radiation emitted by the earth formation, and the tool is guided along the borehole using a wireline. This "total" natural gamma ray vireline logging system was the first type of gamma ray measurement used in geophysical borehole surveying. Since most shale is relatively rich in naturally occurring radioactive elements, these logs are primarily used to draw shale from other formations, or used to measure the shale content of formations. This wireline logging system is used only after the borehole has been drilled.

It is often advantageous to measure total natural gamma radiation while the borehole is being drilled. This is provided by guiding the tool along the borehole using a drill string. This type of system is usually referred to as a total natural gamma ray logging while drilling (LWD) system.

Yet another type of natural gamma ray logging system includes a logging tool that measures a spectrum of gamma radiation emitted by the earth formation. The spectrum is defined as a measure of the intensity of radiation as a function of radiation energy. This type of logging system is usually referred to as a spectral gamma ray logging system. Spectral gamma ray logging tools are typically guided along the borehole using a wireline. Low count rate and detector stabilization are the main problem in any type of natural spectral LWD systems.

Most naturally occurring gamma radiation found in earth formations is emitted by potassium (K) and elements within the decay chains of uranium (U) and thorium (Th). Energy of naturally occurring gamma radiation that can be measured in a borehole environment typically ranges from about 0.1 to less than 3.0 million electron volts (MeV). The elements K, U and Th emit gamma radiation at different characteristic energies. Radiation components from K, U and Th that contribute to the total measured gamma radiation can therefore be provided by identifying these characteristic energies using a spectral gamma ray logging system. By means of system calibration and modelling, these components can then be related to corresponding element concentrations of these elements in the formation. Elemental concentrations of K, U and Th can be used to determine parameters in addition to shale content obtained from total natural gamma-ray logs. These additional parameters include, but are not limited to, clay type, lithology identification, fracture detection, and radioactive tracer location.

As in all nuclear logging systems, statistical precision of a measurement is maximized when the counting rate of the radiation detector used to provide the measurement is maximized. Naturally occurring gamma radiation is typically less intense than gamma radiation induced in formation materials by radiation sources inside a logging tool. It is therefore important to design natural gamma ray tools for maximum measured gamma ray count rate.

Measured count rate can be optimized by designing the tool housing (both total gamma ray and spectral gamma ray) so that gamma radiation attenuation inside the housing is minimized. The lower energy range of the measured spectrum is particularly important in spectral gamma-ray logging systems. Vireline spectral gamma ray logging tools often use a tool housing made of relatively low atomic number material, rather than heavier (and stronger) materials such as steel. These so-called "low Z" toolboxes minimize gamma-ray attenuation, especially at the lower end of the energy spectrum, thereby maximizing the measured count rate for a given radiation intensity and detector size. Low Z materials often do not satisfy structural requirements for LWD systems.

Measure count rates can further be maximized by tool detector design. Due to the relatively high energies of the characteristic K, U and Th gamma radiation, it is advantageous for the gamma ray detector of a given type to be sized as large as practicable to react with, and thereby respond to, these radiations. Typically, larger detectors can be arranged in wireline tools with less damping material between the detector and the formation. LWD systems use a relatively thick tool housing, which typically has a collar with a drilling fluid flow guide passing through the collar. Publication US4698501A relates to the logging of boreholes in the subsurface to determine the formation density using gamma radiation, using a Compton scattering area process.

Publication US5120955A deals with borehole logging, comprehensive correction

of natural gamma ray measurements for borehole fluid effects by calculating a borehole correction factor and applying the correction factor to the calculated element proportion.

A gamma ray detector comprising a scintillation crystal and a cooperating light sensing device, such as a photomultiplier tube, typically provides the highest spectral gamma ray detector efficiency for a given detector volume. Gamma ray detectors undergo significant temperature changes during a logging operation. The gain of a photomultiplier tube changes with temperature and to a lesser extent the count rate changes. Gain changes, often referred to as gain "shifts", adversely affect gamma-ray spectral analysis. It is typical that a 100 degree Celsius (°C) change in temperature causes a 100% change in gain. Temperature variations of this magnitude are not unusual in wireline or LWD logging operations. It is therefore necessary to compensate for detector gain changes in order to provide accurate and precise spectral gamma ray measurements. This compensation is particularly difficult to achieve in LWD systems. As an example, significant gain changes can occur over a relatively short time interval. The data rates of available LWD telemetry systems between the downhole tool and surface equipment are typically too low to effectively monitor and correct for rapidly occurring gain shifts. Automatic down-hole gain control is therefore highly desirable in LWD systems.

As mentioned earlier, naturally occurring gamma ray spectral measurements are typically low count rate. It is therefore desirable to use as much of the measurable gamma ray spectrum as possible in order to maximize statistical precision. Shock and vibration effects on low count rate systems can disturb the spectral shape, especially in the lower energy range of the measured spectrum. This problem is particularly prevalent in LWD systems, which are exposed to rough drilling environments.

The present invention is directed to a spectral gamma ray logging-while-boring (LWD) system. The system is designed to provide elemental concentrations of naturally occurring radioactive material such as K, U and Th. However, it must be understood that the system can be used to provide spectral measurements of any type of gamma radiation experienced in a borehole environment.

The LWD downhole assembly or "tool" comprises a weight tube attached to the lower end of a drill string. A drill bit terminates the lower end of the tool. Sensor, electronics and down-hole telemetry elements are arranged inside the collar. The tool is guided along a wellbore using a drill string, which is driven by a rotating drilling rig on the surface of the earth. Information from the tool is telemetered to the surface via a telemetry link and received by a surface telemetry element contained in surface equipment operatively attached to the drilling rig. Information can also be sent from the surface equipment to the tool via the telemetry link.

The sensor element comprises one or more gamma ray detectors which are arranged as close as practical to the circumference of the tool. This minimizes interfering material between the one or more detectors and the source of gamma radiation, which is the soil formation penetrated by the borehole. As a result of this detector geometry, spectral degradation is minimized and measured count rate is maximized for a given detector size. The detector geometry also allows an azimuth measurement of spectral gamma ray in a plane essentially perpendicular to the axis of the tool. The one or more gamma ray detectors preferably comprise a scintillation crystal which is optically connected to a light-sensitive device such as a photomultiplier tube. The detector element is calibrated under known conditions and to a "standard" detector gain. The sensor element may also contain a system, such as a magnetometer, which senses the orientation of the tool inside the borehole.

Output signals from the sensor element are fed into the electronics element. The signals are amplified using suitable pre-amplification and amplification circuits. Amplified sensor signals are then fed to a processor for subsequent processing. High voltage for the one or more gamma ray detectors is provided by an adjustable high voltage power supply inside the electronics element. Changes in temperature or, to a lesser extent, changes in measured gamma ray count rate result in a detector gain change. Peak structure localization and continuous regions of measured gamma-ray spectra are monitored by the processor. Any gain change is detected using predetermined conditions and criteria stored within the processor. A gain correction signal representative of the magnitude of the gain change is generated by the processor and fed to the adjustable high voltage power supply to thereby adjust the detector high voltage so that the gain is restored to the standard gain. This gain control system is automatic and requires no input from the surface.

With the detector gain stabilized to standard gain, elemental concentrations of K, U and Th are determined in the processor using predetermined conditions. These element concentrations can be fed to the downhole telemetry element and telemetered to the surface. Alternatively, gain-stabilized spectral data can be fed to the down-hole telemetry element and telemetered to the surface for subsequent processing. Spectral gamma radiation data and elemental concentration determinations can be recorded by a data storage device inside the electronics element, and then extracted for processing and analysis when the tool is returned to the Earth's surface.

Elemental concentrations of K, U and Th are determined as a function of depth as the tool is advanced along the borehole. If a number of gamma ray detectors are used, the gain adjusted spectral responses of the detectors are combined to provide the desired element concentrations. The detector responses are preferably combined before calculating element concentrations.

The perimeter of the detector geometry also allows an azimuthal spectral gamma ray measurement and corresponding azimuthal element concentration determination in a plane which is essentially perpendicular to the axis of the tool. The azimuth reference is provided by using a tool orientation sensitive device such as a magnetometer located inside the sensor or electronics element. If a single detector is used, azimuth measurements can be provided only when the tool is rotated by the drill string. A variety of detectors provide azimuth information as the tool rotates or "slides" along the borehole without rotation.

The present invention provides, for adjusting the gain of a gamma ray detector, a gamma ray logging-under-drilling system characterized by the features specified in patent claim 1.

Features of embodiments of the present invention's gamma-ray logging-under-drilling system according to patent claim 1 are stated in patent claims 2-3.

The present invention provides, for adjusting the gain of a gamma ray detector, a method for measuring gamma radiation during drilling of a borehole, characterized by the features stated in patent claim 4.

Features of embodiments of the present invention's method for measuring gamma radiation during drilling of a borehole according to patent claim 4 are stated in patent claims 5-6.

In order that the manner in which the above-mentioned features, advantages and objects of the present invention are provided and can be understood in detail, a more specific description of the invention, which is summarized above, may be obtained with reference to the embodiments thereof which are illustrated in the accompanying drawings , where: Fig. 1 illustrates the spectral natural gamma ray LWD system presented as a whole; Fig. 2 is a functional diagram for main elements and components of the spectral LWD system; Fig. 3 a is a cross section of a spectral LWD tool sensor element comprising a gamma ray detector; Fig. 3b is a side section of the sensor element comprising a gamma ray detector; Fig. 4 is a cross-section of a spectral LWD tool sensor element comprising three gamma ray detectors; Fig. 5 is a typical gamma ray natural gamma ray spectrum measured with the spectral gamma ray LWD tool; Fig. 6 shows a relationship between the slope of the Compton range of the measured gamma ray spectrum and the temperature of the gamma ray detector; Fig. 7 illustrates relationships between detector gain adjustment factor required to provide standard detector gain, a required high voltage adjustment to provide standard detector gain, and detector temperature; Fig. 8 is a more detailed view of a measured gamma ray spectrum and illustrates how a spectral peak position is used to provide a second order detector gain adjustment; Fig. 9 is a graphical illustration of a method for locating statistically significant peak structure in a measured gamma ray spectrum; Fig. 10 is a flow chart showing steps for automatically controlling the gain of a gamma ray detector using a measured spectral analysis method; Fig. 11 illustrates basic concepts used to automatically control the gain of a gamma ray detector using a detector source gain control method; Fig. 12 is a flow chart showing steps for automatically controlling/controlling the gain of a gamma ray detector using the detector source gain control method; Fig. 13 shows additional hardware components necessary to provide natural azimuth gamma ray measurements using the LWD system; Fig. 14 is an example of a spectral gamma ray LWD log showing concentrations of K, U and Th as a function of depth within a borehole; and Fig. 15 is an example of an azimuth spectral gamma ray LWD log and shows concentration of K, U and Th as a function of azimuth around the borehole and as a function of depth inside the borehole.

Details of the preferred implementations of the LWD spectral gamma ray el ogge system are presented in section. System hardware is described first. This is followed by a description of the methodology used to monitor measured gamma ray spectra, and to stabilize the amplification of these spectra as the borehole temperature varies. Two gain stabilization methods are described. With both, stabilization is provided in real time and without operator influence. Once gain stabilization has been achieved, the procedure for determining elemental concentrations of naturally occurring K, U and Th is described. Finally, measurement of total and azimuth concentrations of K, U and Th is described, and "log" presentations of these measurements are illustrated. The invention is directed to the measurement of gamma radiation that occurs naturally in soil formation. However, it must be understood that the basic concepts of the invention can be applied to quantitative measurements of any type of gamma radiation in which one or more gamma ray detectors are exposed to gain shifts.

HARDWARE

Fig. 1 illustrates the LWD system 15 presented as a whole. A downhole unit or "tool" comprises a casing 10 which is attached to the lower end of a drill string 18. A drill bit 11 terminates the lower end of the casing 10. Inside the casing 10 is arranged a sensor element 12, an electronics element 15 and a down-hole telemetry 16. The tool is guided along a wellbore 20, defined by borehole walls 21 and penetrating through formation 22, by means of the drill string 18. The drill string 18 is driven from the surface 24 to the earth by a rotary drilling rig, which is only conceptually illustrated at 26 since such rigs are well known in the art.

Formation from the tool is telemetered to the ground surface 24 via a telemetry link (illustrated conceptually by arrow 23) and received by a surface telemetry element (not shown) contained in surface equipment 28 operatively connected to the drilling rig 26. Information may also be sent from the surface equipment 28 to the tool via the telemetry link 23.

More details of the sensor element 12, the electronics component 14 and the down-hole telemetry element 16 and their operational conditions are shown in the functional diagram of fig. 2. The sensor element 12, which is illustrated conceptually as a box of broken lines, comprises at least one gamma ray detector comprising a scintillation crystal 30 and an optically coupled photomultiplier tube 32. Output signals from the photomultiplier tube are fed to the electronics element, the components of which are enclosed by the box of broken lines, indicated as 14. The signals are amplified using suitable preamplification and amplification circuits 34. Amplified sensor signals are fed to a processor 38. Voltage for the photomultiplier tube 32 is provided by an adjustable high voltage power supply 36 inside the electronics element 14.

Still referring to fig. 2, the processor 38 provides means for automatically controlling the gain of at least one gamma ray detector, and is also used to process signals from the gamma ray detector to provide elemental concentrations of K, U and Th. As mentioned earlier, the tool is preferably calibrated on the surface to a "standard" gain. During logging, the temperature of the tool and its elements change. Temperature changes or, to a lesser extent, changes in measured gamma ray count amounts result in detector gain change. Gain changes are reflected in the energy spectrum of the measured detector signals. Measured features of the spectrum are used to correct for these gain changes, as will be described in detail in subsequent sections of the description. A gain correction signal representative of the magnitude of the gain change is generated by the processor 38 and fed to the adjustable high voltage power supply 36 to thereby adjust the detector high voltage so that the gain is restored to the "standard" gain. Elemental concentrations of K, U and Th are determined from gain-corrected detector spectra in the processor 38 using predetermined conditions as will be described below. Elemental concentrations of K, U and Th are fed to the downhole telemetry element 16 and telemetered, via the telemetry link 23, to the surface telemetry element contained in surface equipment 28.

Fig. 3a and 3b are respectively cross-sections and side-sections of the neck tube 10 in the area of the sensor element and show a sensor element comprising a gamma ray detector. Fig. 3a shows the cross-section A-A' of the collar 10 with the axis of a drilling fluid conductor 44 offset from the axis of the collar. A detector channel on the circumference of the neck tube 10 and defined by the surfaces 40 receives the gamma ray detector comprising a scintillation crystal 30 such as Nal, Csl, BGO and the like. Scintillation crystal 30 is enclosed in a hermetically sealed, light-reflecting enclosure 42. The volume 46 is preferably filled with a material such as RTV, epoxy and the like. The weight tube 10 is surrounded by a thin sleeve 48 in the area of the sensor element 12. Fig. 3b shows the side section B-B' which includes the main axis of the weight tube 10. A photomultiplier tube 32 is optically connected to the scintillation crystal 30. For the sake of clarity, electrical lines to the photomultiplier tube are not shown.

Reference is again made to both fig. 3a and 3b, where it is apparent that the scintillation crystal 30 is arranged as close as practicable to the circumference of the collar 10. This minimizes the intervening material between the detector and the source of gamma radiation, which is the earth formation penetrated by the borehole (not shown). By displacing the axis of the flow conductor 44, the diameter, and thus the efficiency, of the detector is maximized. For typical LWD equipment, the diameter of the scintillation crystal can be 5.1 cm (2") or larger and still maintain the structural specifications of the throat tube 10. As a result of this detector geometry, gamma ray spectral degradation is minimized and measured count quantity is maximized for a given detector size.

Fig. 4 illustrates a sensor element comprising three gamma ray detectors. This cross-sectional view shows the scintillation crystals 30 of each detector. Each crystal 30 is encased in hermetically sealed, light-reflecting casing 42, and is arranged in a detector channel defined by the surfaces 40. The channels are arranged at 120 degrees angular distance. The weight tube 10 is in the area of the sensor element and is again surrounded by a thin sleeve 48. A side section is omitted for brevity, but a photomultiplier tube (not shown) is again optically coupled to each scintillation crystal 30. As with the simple detector sensor element shown in Fig. 3a and 3b, it is apparent that the scintillation crystals 30 are arranged as close as practicable to the perimeter of the tool to thereby minimize intervening material between the detectors and the source of gamma radiation within the earth formation. Using the multiple detector configuration, the axis of the flow guide 44 is coincident with the axis of the throat tube 10. For typical LWD equipment, the diameter of each scintillation crystal is limited to approximately 3.8 cm (1.5") so that structural specifications of the throat tube can be maintained. Compared with the single scintillation crystal sensor element configuration shown in Figures 3a and 3b, the efficiency of each detector is reduced in Figure 4. However, signals from each detector can be combined to provide a total sensor element efficiency equal to or exceeding the efficiency of the single detector configuration. In addition, the three-detector sensor element configuration is benefited by azimuth spectral gamma ray measurements which will be described in subsequent sections of this description.

It should be understood that the multiple detector sensor element configuration is not limited to the three detector configuration shown in FIG. 4. It must also be understood that the angular distance between the multiple detectors need not be equal. Finally, it must be understood that the dimensions of the multiple detectors do not have to be the same.

GAIN STABILIZATION

Two methods for detector gain stabilization are described. The first method will be referred to as the "measured spectral analysis" method, and the second method will be referred to as the "detector source" gain correction method. The gain of an LWD gamma ray detector can change significantly and rapidly in the rough borehole drilling environment. Telemetry links between the tool and the surface are relatively slow and do not allow gain monitoring and correction from the surface. Gain control or management must be implemented automatically inside the tool. Both of the methods described can be used to effectively control/manage gamma ray detector gain.

Taking into account the importance of gain control and the roughness of the borehole environment, it is desirable to use both methods. The two methods may be used with one serving as a primary gain control method and the other serving as a backup gain control method. Alternatively, both methods can be used simultaneously and the results combined to provide a detector gain correction. Such a combination can take a number of forms including a simple numerical average or a weighted average.

MEASURED SPECTRAL ANALYSIS PROCEDURE

Fig. 5 is a typical natural gamma ray spectrum measured in earth formations with a scintillation type gamma ray detector. The spectrum comprises measured gamma-ray intensity as a function of gamma-ray energy, represented by curve 50. The abscissa is gamma-ray energy in million electron volts (MeV), and the ordinate in the natural logarithm of measured count amount is energy increment. The increments of energy are represented by energy channels or "channels" on the top scale abscissa. Representative peak structures from K, U and Th are shown at respective energies 1.46 MeV, 1.76 MeV and 2.61 MeV. During instrument calibration, the detector high voltage is adjusted to provide a detector gain for which specific energies of gamma radiation fall within predetermined energy channels. This gain is referred to as the "standard" gain. Tool calibration will be described further in a subsequent section of this description.

The Compton scatter region of the spectrum includes formation gamma radiation that has undergone multiple collisions in intervening material before reaching the gamma ray detector. This region of the spectrum is identified by the reference number 52 and is terminated at the low energy region by the "hump" 54 at energy 56. This exponential-looking region 52 contains no direct contributions from K, U and Th gamma radiation. The slope of this range is mainly a function of the photomultiplier tube gain and can be used to monitor detector gain.

Fig. 6 is a plot of the measurements of the slope of the Compton range 52 as a function of detector temperature in degrees Celsius (°C). Measured values for slope (ordinate) versus temperature (abscissa) are indicated by the data points 62. A curve 60 fitted through the data points 62 shows that there is an essentially 1:1 correlation between the slope and detector temperature.

Curve 66 in fig. 7 shows the relationship between a multiplicative first order gain adjustment factor Fi and corresponding detector temperature, where

G0bs the observed detector gain, and Gstder the previously defined "default" gain for which the tool is calibrated. Relationship between slope versus temperature shown graphically as curve 60 in fig. 6 can be combined with gain adjustment factor as a function of temperature shown graphically as curve 66 in FIG. 7 to eliminate the temperature parameter. This combination provides a functional relationship between the measured slope and the desired first order gain adjustment factor Fi.

At this point, detector high voltage can be adjusted to correct detector gain for temperature effects. Curve 68 in fig. 7 shows the relationship between required high voltage adjustment to provide Gstdog detector temperature. Once Fi has been provided as described above, the high voltage Vi required to provide Gstdb can be determined. The following example is represented as a graphical solution. Suppose that from a measurement of the Compton slope, it has been determined that Fi = 1.1 as indicated in Fig. 7 at 70. A horizontal line is projected until it crosses curve 66 at a point 71. A vertical line is projected until it crosses curve 68 at point 72. Finally, a horizontal line is projected to the right ordinate at point 73 and provides a necessary high voltage correction of +10 volts. Reference is again made to fig. 2 where the 10 volts added to the photo-multiplier tube 32 from the adjustable high voltage power supply will adjust the gain of the gamma ray detector to the standard gain Gstd.

To summarize, the conditions are shown graphically in fig. 6 and 7 combined to develop a ratio of measured slope in the Compton range as a function of high voltage required to maintain standard detector gain. The example described above is graphical, but it should be understood that the solution can be reduced to analytical form suitable for calculation in a processor. A measure of slope can therefore be used to correct for detector gain offsets. Using this slope to predict gain changes is extremely robust since it is calculated from a number of data points, is immune to electronics noise, and has been found not to be greatly affected by wellbore and formation conditions. A first order detector gain correction can be performed for most severe gain changes in the range from about -60% of standard gain to +150% of standard gain. Such severe gain changes are induced by equally severe changes in temperature in the range from about -60°C to about +150°C, where 25°C is the "standard" calibration temperature.

As described above, a measure of slope can be used to provide a voltage adjustment necessary to provide standard gain. This is a first order correction taking into account the magnitude of gain changes that can be handled. Second and third order corrections are performed to further increase the precision and accuracy of the detector gain settings. Because of these additional corrections, it is preferred not to adjust detector voltage after the first order gain correction. Instead, the spectrum is adjusted by adjusting the count amount per energy channel using the first order gain correction factor Fi. Using the earlier example of Fi = 1.1, the width of each channel is "stretched" 10 percent, and the measured counts are redistributed over the wider channels. This methodology can be thought of as a "software" gain shift, and is preferably performed in processor 38. The detector gain adjustment is now further refined by examining a predominant peak in this modified energy spectrum. Fig. 8 shows a part of the full spectrum 50 shown in fig. 5, and includes a peak 80 at 1.46 MeV from K. This peak is typically the most prominent peak in the spectrum, and is suitable for the second-order gain adjustment. After modifying the spectrum using the methodology described earlier, it is observed that the maximum to the peak 80 falls in a channel Ch0bsved 78. Instrument calibration requires that the energy corresponding to this maximum peak falls in a modified channel Chjjtdved 76, where the channels have been aligned in width with the first-order correction Fi. The gain of the detector is further adjusted to a second order correction so that the maximum to peak 80 falls in Chstd. This is again provided with a "software" gain shift by adjusting the widths of the energy channels and redistributing the measured counts to form a second modified spectrum. A second order gain correction F2 is provided from the equation The gain correction process can be terminated at this point without any further refinement of the gain setting. If this option is selected, the corresponding high voltage setting V2 required to provide this second order corrected standard gain is expressed mathematically as

Peaks in measured gamma ray spectra such as the curve 50 shown in fig. 5 is identified with respect to the channel (and corresponding energy) using another derivative algorithm after a strong filter is applied. Locations of the peaks are determined using a Gaussian curve fit around each peak. Fig. 9 illustrates the efficiency of the detection and location method. Curve 86 represents the second derivative d<2>C/dCh<2> (right ordinate) of the spectral count quantity C as a function of corresponding energy channel Ch for a spectrum of the type shown in fig. 5. Peaks are indicated when the curve 86 crosses d<2>C/dCh<2>= 0. Since the spectrum has statistical variations, the curve may also have a "zero crossing" due to count quantity statistics rather than actual peak structure. It is therefore necessary to identify a non-statistical or "noise" zero crossing from a true zero crossing indicating a peak. curves 82 and 84 represent upper and lower bounds for standard deviation of count quantity C as a function of energy channel, and are presented in arbitrary count quantity units to the left ordinate. Only zero crossings that extend outside the standard deviation "envelope" are considered statistically significant indications of a peak. There is only one such indication in curve 86 at a channel identified by reference number 88. This corresponds to the K peak at 1.46 MeV shown clearly in fig. 5.

It is preferred to further refine the gain setting. Once the second modified energy spectrum has been calculated, all statistically significant peaks in the second modified spectrum are located using the peak location technique described above. Once the energy channels in which these peaks are observed, they are compared to their corresponding "standard" channels determined by instrument calibration. This methodology is similar to the single-peak methodology used for the second-order gain correction, but all peaks are used in this third-order correction. Channel widths are again adjusted and count amounts redistributed so that all identifiable peaks fall in their corresponding standard energy channels, corrected for the first and second order software gain adjustments. This is the third order gain correction and gives a third order gain adjustment factor F3. The uncorrected detector voltage V is now adjusted to provide a corrected voltage Vcor using the relationship The corrected detector gain Gcorer

where H is a multiplicative constant relating the third order s corrected high voltage Vcor to the fully corrected detector gain factor Gcor. Channel widths are reset to their original values.

The measured spectral analysis procedure for automatic gain correction is summarized in the flow diagram shown in Fig. 10. The slope of the Compton area 52 (see fig.

5) is measured at 90. The first order gain correction Fi is determined at 92 using the measured slope as described above. An identifiable peak is located in the measured spectrum at 94. The second order gain correction F2 is determined at 96 using previously described methodology and equation (2a). All statistically significant peaks in the measured gamma ray spectrum are determined at 98. The third order gain correction factor F3 is provided at 100 by a software gain adjustment that positions all peaks flush with their associated energy channels provided by instrument calibration. The corresponding high voltage Vcorsom required for standard detector gain is also determined at 100 using equation (3a). The corrected detector gain Gcor is set at 102 using V3 and equation (3b). It is again noted that a real voltage adjustment is made only after the third order correction, where software gain adjustments are used in the first and second order corrections. It should also be understood that other algorithms may be used to provide suitable software gain adjustments for the first and second order corrections.

DETECTOR SOURCE GAIN CORRECTION METHOD

Gamma ray detector gain can be monitored using an alternative technique. A small radioactive "detector" source is located near or inside the one or more scintillation crystals and comprises the natural gamma ray LWD sensor element. The detector source generates a "calibration" peak in the measured gamma-ray spectrum. If the gain of the measured spectrum changes, the position of the calibration peak shifts with the change in gain. A measure of the position of the calibration peak can therefore be used to monitor and correct detector gain.

The calibration peak is preferably at a relatively low energy so that it will not interfere with higher energy radiation from K, U and Th which is used to determine element concentrations. A suitable detector wedge is Americium-241 (<241>Am) which emits gamma radiation at 0.060 MeV. Reference is again made to fig. 5, where a typical measured gamma ray spectrum encompasses an energy range from 0.0 to 3.0 MeV over typically 256 channels. With this "default" detector gain setting, the low energy end of the spectrum, which includes the 0.060 MeV calibration peak, is very susceptible to electronics noise. Additionally, since the spectrum spreads over about 3.0 MeV and typically 256 energy channels, the<241>Am peak occupies only about 3 of the 256 channels, making it difficult to precisely locate the peak position.

Attention is drawn to fig. 11 which conceptually shows the low energy region of a gamma ray spectrum measured with a detector comprising a<241>Am detector source. Curve 110 shows the peak structure with the detector set to standard gain. Since the peak occupies three energy channels, it is difficult to locate the center of the peak using previously described methods. This peak can in principle be used as shown to stabilize detector gain. Any stabilization using the<241>Am peak at 0.060 MeV will therefore be prone to large errors, especially in the higher energy range of the spectrum used for element concentration calculations.

The accuracy of detector gain stabilization using a low energy detector source and calibration peak is improved by the use of dual gain circuits. The spectrum signal from the gamma ray detector is "branched off" and fed into first and second amplification circuits comprising the dual amplification circuits. The first circuit comprises a standard gain circuit and generates a standard gain spectrum. The 0.060 MeV 241Am peak in this standard spectrum is shown at 110 in Fig. 11. For gain stabilization purposes, the spectrum signal is fed to a second amplification circuit with a gain factor N greater than the gain factor of the standard gain. The second amplification circuit generates an amplified spectrum with a "boosted" gain. For purposes of description, it will be assumed that N = 10, although it should be understood that other values of N may be used. Curve 112 is the amplified gain spectrum showing the<241>Am peak at 0.060 MeV amplified by a factor of N = 10. The peak now occupies approximately 30 energy channels, and previously described peak localization procedures are used to determine that the center of the peak is in energy channel P0bs, as identified at 114. From tool calibration it is known that energy 0.060 MeV should fall into energy channel Pstd for the standard gain, or in channel N x Pstd for the enhanced gain, shown at 116 in fig. 11. The detector high voltage V is adjusted to a corrected value, Vcor, using this relationship A signal proportional to (N Pstd/P0bs) is preferably generated in the processor 38 and fed to the adjustable high voltage power supply 36. This generates the corrected high voltage Vcor lever to the detector. Corrected standard gain-gain Gcorer expressed by the ratio

where H, as in equation (3), is a multiplier constant relating high voltage to detector gain.

It should be understood that various methods may be used to increase the detector gain by a factor of N. As an example, the gain circuit 34 (see FIG. 2) may contain a high gain element which, under the control of the processor 38, "branches" the detector input. The gain of one branch of the input by a factor of N thereby forms the amplified gain of the calibration peak for purposes of stabilizing calibration source gain.

The gain stabilization method using a radioactive "detector" source is summarized in the flow diagram of FIG. 12. The low energy portion of the measured gamma ray spectrum is increased by a factor of N at 120. The energy channel P0bs, in which the stabilization peak is maximum, is determined at 122 using a suitable peak location technique. High voltage Vcorsom required to position the peak maximum value in the energy channel N x Pstdb is determined (see equation (4)) at 124, and the correct detector gain Gcor is set at 126 (see equation (5)).

Hybrid correction procedure

The measured spectral analysis method and the detector source gain correction methods can be combined to provide a hybrid gain control method. A calibration source is arranged inside or in the immediate vicinity of at least one gamma ray detector. When in the borehole, this detector produces a gamma ray spectrum comprising a first component from naturally occurring radioactive elements within the formation and a second component from the calibration source. A first detector gain correction is determined from spectral features of the naturally occurring gamma radiation as previously described in the measured spectral analysis procedure. The second gain correction is determined from the calibration source component as previously described in the detector source gain correction procedure. The first and second gain corrections are combined to correct for gain drift in the detector.

ELEMENT CONCENTRATION PROVISIONS

With detector gain stabilized to "standard" gain, elemental concentrations of K, U and Th are determined, preferably in the processor 38 of the electronics element 14 (see Figs. 1 and 2), from measured spectral data. These element concentrations can be fed into the down-hole telemetry element 16 and telemetered via the telemetry link 23 to the surface equipment 28. Alternatively, the spectral data can be fed into the down-hole telemetry element 16 and telemetered to the surface equipment 28 for subsequent processing. Since the telemetry bandwidth is limited and the gamma ray spectrum is much more data-intensive than the element concentrations determined by these, it is preferred to telemeter the element concentration of K, U and Th to the surface. Alternatively, spectral gamma ray data and element concentration determinations can be recorded by a data storage device inside the electronics element and then extracted for processing and analysis when the tool is returned to the surface.

The following methodology is preferred for determining elemental concentrations of K, U and Th. However, it should be understood that other spectral processing methods such as spectrum stripping, peak area analysis and the like may be used to determine concentrations of K, U and Th. The necessary element concentration calibration constants are provided by tool calibration.

Elemental concentrations are provided by solving the matrix equation

[C] is an m x 1 column matrix comprising elements q (i = 1,m) representing count rate registered in energy channel i (see fig. 5). It is typical that 256 energy channels (m = 256) are used, although more or fewer channels may be used within the scope of the invention.

[A] is an m x j matrix comprising elements aijmed (i = 1,m) and (j = K, U, Th). Physically, the element aij is the sensitivity of energy channel i to the element j, typically in counting units per second per part per million (U and Th) or counts per second per percent (K). The matrix [A] comprises calibration constants, and is referred to as a

"sensitivity" matrix, and is determined by tool calibration. In tool calibration, the response of the tool is measured in formations containing known concentrations of K, U and Th, and in "standard" borehole environments, and with one or more detectors in the sensor element operated at "standard" gain Gcor.

[M] is a j x 1 column matrix comprising elements Mj (j = K, U, Th) which are the parameters of interest, namely the formation's elemental concentrations of K, U and Th. MK is in percentage, and Mu and Mxhi share per million (ppm). The desired element concentrations are provided by solving equation (6) for [M], preferably using a weighted least squares fit.

Measured gamma ray spectra from one or more gamma ray detectors in the sensor section are tracked as a function of depth to the tool in the borehole 20 (see Fig. 1). If the sensor element 12 only comprises a gamma ray detector as shown in fig. 3a and 3b, the elements of matrix [C] are provided from this detector. If the sensor element comprised a number of q detectors, such as q = 3, the design shown in fig. 4, the elements of the [C] matrix are provided by combining responses from the q detectors, typically by simply summing the responses if all detectors have equal sensitivity.

AZIMUTE ELEMENT CONCENTRATION DETERMINATIONS

The spectral gamma ray LWD system can be used to measure elemental concentrations MK, Mu and Mxh as a function of downhole azimuth as well as a function of downhole depth. Azimuthal measurements require additional components arranged preferably inside the electronics element 14. Fig. 13 is a functional diagram for components added to the electronics element shown in fig. 2 so that azimuthal element concentrations MK, Mu and Mxhkan are determined. A device that senses tool orientation, such as a magnetometer 130, and a clock 132 is operatively connected to the processor 38. As in the previous description of fig. 2, signals from the one or more gamma ray detectors and amplifier circuits are fed to the processor at 136. The processor 38 in turn controls detector gain adjustments to the detectors at 138 using the adjustable high voltage power supply 36. Spectral and elemental concentrations are output from the processor 134 as described below.

As the tool is rotated 360 degrees, the gamma ray spectrum with the shape is displayed

fig. 5 measured during individual time intervals At, where these elements are defined by the clock 132 which cooperates with the processor 38. Spectra are stored in storage locations in accordance with time intervals At in which they are measured. The time interval At is preferably about 50 milliseconds. During each time interval, the average reading of the magnetometer 130 is determined and thereby defines an azimuth sector assigned to each time interval, thereby assigning an azimuth sector to each storage location. Each stockpile contains a gamma ray spectrum measured in a known borehole azimuth sector. The spectral data stores and the average of the magnetometer readings during each time interval are controlled by the processor 38. Stored spectral data and corresponding azimuth sectors are preferably stored in the processor 38. The process is repeated over multiple 360 degree rotations within a given depth interval Ad in order to maximize statistical precision for each natural gamma ray spectrum stored in each storage space. Gain stabilization techniques previously described in detail are used to control the gain of each stored spectrum. Previously described data analysis procedures are used to calculate the matrix [M] for each stored spectrum and thereby provide elemental concentrations Mk, Mu and Mxh for each azimuth sector around the borehole.

If the sensor element comprises a plurality of detectors, detector output signals are phased by the processor 38 so that as each detector rotates through each azimuth sector, output signals from this detector are stored in the storage space corresponding to this azimuth sector.

The tool can be guided along the borehole without rotation. This guidance is usually referred to as "gliding". If the sensor element 12 comprises only a gamma ray detector, azimuthal natural gamma ray spectral measurements cannot be performed when the tool slides. If the sensor element comprises a plurality of gamma ray detectors, azimuthal spectral measurements can be provided during gliding. The sizes of the azimuth sectors are determined by the number of detectors in the sensor element. For the sensor element comprising three detectors at 120 degree centers as shown in fig. 4, each azimuth sector will be 120 degrees. This provides an azimuthal resolution that is typically inferior to that provided with rotating tools and with time intervals Åt of approximately 50 milliseconds.

LOG PRESENTATIONS

Fig. 14 shows an example 140 of a natural gamma ray LWD log presentation of Mk, Mu and Mxh as a function of borehole depth. The quantities Mk, Mu and Mxh are calculated from measured spectral data [C] using equation (6). Units for concentrations of K (%), U (ppm) and Th (ppm) are shown in the respective fields 141, 143 and 145. Scales are typically in % per map division for K and ppm per map division for U and Th. Concentrations of Mk, Mu, and Mx are shown as a function of depth 148 in the borehole by the respective curves 142, 144, and 146. As an example, outliers 147 and 149 in Mk and Mu, respectively, are indicated at a depth of approximately xx20. A strike 159 in Mxher indicated at a depth of about xx40. It should be understood that other formats may be used to present the basic LWD natural gamma-ray log data. Fig. 15 shows an example 150 of an azimuthal natural gamma ray LWD log presentation. Elemental concentrations of MK (%), Mu (ppm) and Mxh(ppm) are indicated by solid, long-dashed and short-dashed curves 164, 162 and 160, respectively, as shown in field 151. Corresponding scales for these concentrations are tabulated in field 150 and are typical in % per map division for MK and ppm per map division for Mu and Mxh Concentrations MK, Mu and Mxh, provided from spectra summed over a depth interval Ad shown in field 153, are shown as a function of azimuth sector 152, in degrees, for this depth interval. As an example, outcrops 156 and 154 in MK and Mu, respectively, are shown at an azimuth sector of approximately 180 degrees over the depth interval xx20. As another example, a strike 158 is in Mxhvist at an azimuth sector of about 225 degrees over the depth interval xx40. It should be understood that other formats may be used to present the basic LWD azimuthal natural gamma-ray log data.

While the foregoing description is directed to the preferred embodiments of the invention, the scope of the invention is defined by the claims, which follow.

Claims (6)

1. Gamma-ray logging-during-drilling system, characterized in that it comprises: at least one gamma-ray detector that measures a gamma-ray energy spectrum; wherein the system is configured to perform a first adjustment of gain to said detector using measures of slope to a Compton scatter region of said spectrum. 2. System according to claim 1, characterized in that the system is configured to: (a) measure the location of an energy peak in said spectrum; and (b) performing a second adjustment of said gain such that said location corresponds to a standard location for said energy peak. 3. System according to claim 2, characterized in that the system is configured to: (a) measure locations for a number of energy peaks in said spectrum; and (b) performing a third adjustment of said gain such that each said location of each of said plurality of peaks corresponds to a standard location for said peak. 4. Method for measuring gamma radiation while drilling a borehole, characterized in that it comprises: (a) measuring a gamma ray energy spectrum with at least one gamma ray detector; and (b) making a first adjustment of gain to said detector using a measure of slope to a Compton scatter region of said spectrum. 5. Method according to claim 4, characterized in that it comprises the further step of making a second adjustment of said gain by: (a) measuring the location of an energy peak in said spectrum; and (b) adjusting said gain so that said location corresponds to a standard location for said energy peak. 6. Method according to claim 5, characterized in that it comprises the further step of making a third adjustment of said gain by: (a) measuring the location of a number of energy peaks in said spectrum; and (b) adjusting said gain so that each said location of each of said plurality of peaks corresponds to a standard location for that peak.